With equipment of this kind, use is generally made of test tubes that may be of various shapes and sizes. To achieve a good level of automation and eliminate all sources of error, it is advisable to carry out a number of checks, especially checking that test tubes are present and that they have been correctly positioned in the instrument. In addition, it is always advisable to check the efficiency of the optical systems (emitter+receiver), so that errors of insertion and position of the test tubes are not inadvertently made.